In accordance with a rapid spread of information relevant apparatuses and communication apparatuses such as a personal computer, a video camera and a portable telephone in recent years, the development of a battery to be utilized as a power source thereof has been emphasized. The development of a high-output and high-capacity battery for an electric vehicle or a hybrid automobile has been advanced also in the technical industries such as the automobile industry. A lithium battery has been presently attracting attentions from the viewpoint of a high energy density among various kinds of batteries.
Organic liquid electrolyte having a flammable organic solvent as a solvent thereof is used for a presently commercialized lithium battery, so that the installation of a safety device for restraining temperature rise during a short circuit and the improvement in structure and material for preventing the short circuit are necessary therefor.
On the contrary, an all solid lithium battery all-solidified by replacing the liquid electrolyte with a solid electrolyte has the advantages of attaining the simplification of the safety device and being excellent in production cost and productivity for the reason that the flammable organic solvent is not used in the battery. A sulfide solid electrolyte has been conventionally known as a solid electrolyte used for such an all solid lithium battery.
However, the problem is that the sulfide solid electrolyte is low in stability against moisture. Thus, a method for synthesizing the sulfide solid electrolyte on the conditions of less moisture has been conventionally known. For example, a method for synthesizing the sulfide solid electrolyte, in which heating and melting are performed in an inert gas stream with a moisture amount of 100 ppm or less, is disclosed in Patent Document 1.
Among the sulfide solid electrolytes, Li7P3S11 is so high in lithium ion conductivity as to be expected as a useful material for an all solid lithium battery. A crystal structure of Li7P3S11 is disclosed in Nonpatent Document 1; specifically, a crystal structure, in which a P2S7 unit including cross-linking sulfur (a unit represented by the structural formula B mentioned below) and a PS4 unit having no cross-linking sulfur (a unit represented by the structural formula C mentioned below) are arrayed at a ratio of 1:1 is disclosed therein. Li7P3S11 may be synthesized by using a material composition prepared so as to satisfy a relation of Li2S:P2S5=70:30 on a molar basis.
The synthesizing of the sulfide solid electrolyte by using a material composition prepared so as to satisfy a relation of Li2S:P2S5:P2O5=70:30-x:x on a molar basis is disclosed in Nonpatent Document 2. This technique allows the sulfide solid electrolyte with electrical stability improved.    Patent Document 1: Japanese Patent Application publication No. 6-279050    Nonpatent Document 1: H. Yamane et al., “Crystal structure of a superionic conductor Li7P3S11”, Solid State Ionics 178 (2007) 1163-1167    Nonpatent Document 2: K. Minami et al., “Structure and properties of the 70Li2S-(30-x)P2S5-xP2O5 oxysulfide glasses and glass-ceramics”, Journal of Non-Crystalline Solids 354 (2008) 370-373